Carlos Sainz Jr says McLaren's struggles in the part of the Barcelona Formula 1 track most relevant to Monaco were accentuated by a problem that only emerged in testing.

McLaren had hoped that a major upgrade it introduced for the Spanish Grand Prix would put it at the forefront of the tight midfield pack, but neither Sainz nor team-mate Lando Norris made it through to Q3 at Barcelona.

Swift pitstops and a good strategy call during the late-race safety car helped Sainz grab points in eighth, but the team was still mindful that it had not been quick enough in the tight third sector in Spain - which traditionally offers a good indicator of expected Monaco form.

But concerns for Monte Carlo have been eased by discoveries McLaren made in the Spanish GP post-race test.

"In Barcelona, we learned a lot from that test," explained Sainz.

"We obviously analysed what happened in qualifying and the race, and we understand why particularly in our case we were missing a lot of performance in the last sector.

"We found an issue, we found a problem, and hopefully it will bring us to good spirits here knowing that the car was not at 100% back at Barcelona."

Pushed on whether the discovery was linked to a wrong set-up or something actually broken, Sainz did not wish to elaborate.

"I cannot go too much into detail," he said. "There was something not working as it should."

The discovery of the Spanish GP problem has left Sainz more hopeful for McLaren's prospects for Monaco, although he admitted it was hard to predict how its car would fare based on this season's variable form.

"It's another one of those tracks that has been good for me in the past," said Sainz, who has scored points in Monaco on every F1 appearance so far - including coming from 20th to 10th for Toro Rosso in his rookie season in 2015 and qualifying and finishing sixth for the team two years later (pictured below).

"There is a bit of a question mark this year more than others due to this tight midfield and this big [swing] of performance that we have from China to Baku to Barcelona.

"Here there is not one solid midfield team dominating at the moment and that probably makes us wonder what will happen in Monaco.

"We've been consistent, we've been doing some good Sundays lately so hopefully we can keep that up.

"The low speed hasn't been our strength this season, so then you would say 'why are you strong in Bahrain?' Bahrain there were a lot of long straights and hairpins where we know we are good. But consecutive low speed corners, we haven't been at our best.

"We can see China, long corners, we were not at our best. That is why Monaco is a question mark. It is short, sharp corners, but it is low speed so we don't know if we will behave like in Bahrain or like in Barcelona last sector.

"That is a big question mark for us and hopefully we will know on Thursday. In FP2. I will start to know what we are looking at."

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